Discovery of 3-O-β-chacotriosyl oleanane-type triterpenes as H5N1 entry inhibitors†
A series of 3-O-β-chacotriosyl oleanane-type triterpenes have been designed, synthesized and evaluated as H5N1 entry inhibitors, based on a small molecule inhibitor, saponin 1, previously discovered by us. Detailed structure–activity relationship (SAR) studies on the aglycone of compound 1 indicated that oleanane-type triterpenes with conserved structural features as the aglycone favored the antiviral activity. The results suggested that the introduction of bulky groups onto the 28-COOH of OA was helpful for significantly improving the selectivity index while keeping the antiviral activities, which was opposite to what was found for the GA analogs. Compound 5 was selected for further mechanistic studies because of its distinguished inhibition activity and good selectivity index. Molecular simulation analysis confirmed that compound 5 stabilized the HA2 subunit of hemagglutinin (HA) by binding with amino acid residues THR-189, LYS-156, and ARG-133A, and therefore may prevent HA from undergoing conformational rearrangement, which is a critical step for viral entry.